Study and application of CBM static and flowing material balance methods

Water and gas production law in coal bed reservoir is different from that of conventional containing-water gas reservoir, and gas content change is affected by Langmuir pressure and desorption rate, which bring difficulty to accurate reservoir evaluation.Through the literature research, it was found that the material balance method was widely used in conventional reservoirs, greatly more than that in unconventional ones.On the basis of previous studies, the paper has divided typical production stage, and set up corresponding production model in terms of CBM fluid output law.Meanwhile, static and flowing material balance models were derived and set up to determine single well controlled reserves and permeability by considering the influence of adsorption and desorption characteristics and using bottomhole flowing pressure and cumulative output etc.The research results showed that connate water in coal reservoir pore cleet was finite, high content at initial stage and later weakening; coal bed physical property was featured with falling first and then rising in the process of production, which was affected by stress sensitivity and matrix shrinkage effects.Static and flowing material balance methods can calculate the single well dynamic controlled reserves, but flowing material balance method can avoid the harm of shut-in pressure measurement to the recovery of reservoir productivity, and obtain the dynamic reservoir permeability.Both flowing and static material balance methods have similar calculating results.The research is of great significance to the calculation of controlled reserves and evaluation of physical property.